In the story I am writing right now, a scene portrays the attempt to kill the main character in his own apartment. After having studied the floor plan, the assassin decides to shoot him from the neighboring flat - through a wall. Here is the full plan:

The watered closet of the target's apartment is adjacent to a small, windowless room in the neighboring apartment that allows for safe, stealth operation. The wall separating the toilet from said room is fairly thin.

After gaining access to the neighboring apartment and quietly neutralizing its occupant by means of chloroform, the assassin enters the room. He carries his equipment in a sports bag.

The assassin listens for the flush of the toilet so that he can locate the seat.

After that, he uses chalk to mark a spot on the wall roughly corresponding to where the upper torso of the target would be when sitting.

The assassin waits for the next toilet session of the victim (he has previously put a diarrhea-causing substance in his food). As soon as he hears footsteps and the clanking of the toilet lid and confirms that the target is seated, he aims at the marked spot and shoots through the wall.

He then uses the hole created in the process to check up on the victim and/or administer more shots in the case of survival.

18 Answers
18

It might work as written, but has a simple issue: you specify this is an apartment building.

This means that odds are high that the toilet room wall is what's called a "wet-wall" in construction and building design: they stack the toilet rooms, floor over floor, and organize the piping so that it runs vertically through the thickened wall or through a chase, typically directly behind the toilet.

It's therefore probable that there are not only pipes, but likely multiple quite large pipes running through that wall: water supply pipes (large), waste outlet pipes (larger), and stack vents (smaller) - the stack vents run all the way to the roof (or sometimes have wall outlets depending upon locality) and all others run from top floor down to lowest level where they connect to the mains.

These pipes are typically passing through floor-ceiling assemblies with fire caulking and other fire and smoke suppression sealants and systems, meaning there's little sound transmission between floors or along pipes, other than water hammer sounds inside the pipes themselves.

To convincingly work with this reality, I suggest:

After the assassin chalk-marks the spot on the wall, they use a hand-auger (old fashioned un-powered slow quiet drill) to bore a hole through the wall covering in the room in which they're waiting, allowing them to visually inspect the piping behind the wall surface to determine the best shot location, and place the muzzle directly into said bored hole, so that when they do fire, the main sound is inside the wall cavity and thus resonates less, being restricted from open air expansion and reverberation, and what sound is propagated will sound enough like water hammer as the pipes will partially resonate to the gunshot to be easily ignored.

$\begingroup$If the complex uses PVC pipes, I suspect the bullet wouldn't be slowed or deflected enough to prevent the assassination, even without the auger.$\endgroup$
– CaMJun 21 '18 at 15:33

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$\begingroup$@CaM - unlikely if the complex is much above 3 stories - the combination of required head pressure and the feed pressure drop with PVC are unacceptable in that scenario. Those will be iron pipes, or possibly copper. The stack vents might be PVC, but many municipalities prevent that through regulation due to the potential for noxious odour buildup; many also prevent the use of PVC on multi-story due to added fire risk and toxic combustion product offgassing in case of fire.$\endgroup$
– GerardFallaJun 21 '18 at 15:50

10

$\begingroup$@BladeWraith Firing a 50 caliber sniper rifle from within a room of unknown size might be problematic$\endgroup$
– Dent7777Jun 21 '18 at 19:06

$\begingroup$"visually inspect the piping behind the wall covering" made my jaw drop - and thereby shows that a lot of the feasibility of the idea also depends on the country it takes place in$\endgroup$
– Hagen von EitzenJun 23 '18 at 22:05

If he can get a laxative to the target, why not a poison? If he can get to the neighbor silently, why not the target? Now if the task is to setup the neighbor using the neighbors own gun in some sort of 'accident', then this would make sense. There needs to be a reason for the complexity behind it.

$\begingroup$Why not just use a sniper rifle through the window from the roof of the building across the road. The OP's plan seem overly complicated.$\endgroup$
– ThorneJun 22 '18 at 6:19

$\begingroup$I am ashamed that I read through all of the other answers and didnt ask myself any of these questions, great points!$\endgroup$
– MarieJun 22 '18 at 17:48

1

$\begingroup$To answer your good question - If he can get a laxative to the target, why not a poison? ... the target eats from a buffet bar. Everybody who eats from the Golden Hog trough will be getting diarrhea but only the target will be killed. A related question is why does the target have to get diarrhea. As a parent, I know that "Everybody poops." and laxatives are not necessary.$\endgroup$
– emoryJun 24 '18 at 1:34

People have already done a good job of answering the realism aspect here, so I'll just focus on weapon choice.

Overall, you have a couple of constraints for this type of scenario:

The weapon has to be compact. Conventional sniper rifles like you would get from PGM, Accuracy International, or other big firearm companies are pretty much out, they're way too long. Even a more conventional rifle is likely to be a bit on the large side here, as they are typically at least two feet long. This largely limits you to three options, a handgun, a bullpup rifle or PDW (one where the chamber is behind the grip, examples include the Steyr AUG and the FN P90), or a military CQB carbine (like the US M4 carbine). Note that this is factoring in the use of a 'silencer', which are usually at least 10 cm long.

If you want it to be quiet, you need a subsonic round. Contrary to popular belief, a 'silencer' does not actually make a gun silent, which is why they are typically instead called suppressors. There are three sources of sound when firing a gun, the detonation of the powder load in the cartridge, the sonic boom from the bullet itself, and the mechanical sounds of the gun operating. A suppressor only takes care of the first source (the sound of the detonation). To deal with the second source, you need a round with a velocity of less than about 300-320 m/s. That immediately eliminates almost all handgun cartridges except .22 LR, .25 ACP, .32 ACP and .45 ACP, and a vast majority of widely available rifle cartridges.

You want something that's not going to over-penetrate your target. Overpenetration, somewhat paradoxically, actually increases the chance of survival in most cases (terminal ballistics gets really weird sometimes). It would also risk hurting someone other than your target. This would normally mean hollow point or soft point rounds, but both are out as options if you're shooting through a wall (the wall will cause them to expand, instead of them expanding in the target as needed. This leaves you with needing something reasonably low power to begin with, or that just naturally tumbles or expands in your target.

Given all of this, you've got a pretty limited set of options if you want something that actually exists today. Modern bullpup designs generally are not very flexible in terms of chambering (and thus will be hard to meet the second and third requirements with), and most handgun rounds that meet the second requirement require being essentially right next to your target and aiming directly at a vital organ like the heart to reliably kill someone (.22 LR will bounce around in the chest cavity though, so that might be a decent option here). An FN P90 might work for this (and would look really cool too), the 5.7x28mm rounds can easily punch through a wall (the military loads can punch through NATO issue body armor at 100 meters without issue), will tumble inside the target (this is a side effect of the shape of the bullet), and are not particularly loud (they are supersonic, but it might be possible to do a custom subsonic load that works with a suppressor, and even if not it won't likely be loud enough to cause trouble), and the gun itself is really compact.

Realistically though, the most likely option if the assassin is well funded is going to be a custom made weapon chambered in something like .300 BLK (a rifle round designed for optimal subsonic performance and good ballistics with a suppressor), either in a bullpup arrangement, or built similarly to a pistol-configuration AR-15 (that is, one with no stock and a short barrel).

$\begingroup$Also, even if the gun would be as silent as suppressed guns are presented in most movies (totally impossible, but bear with me), punching a hole in the wall will be loud enough for everyone in the apartment complex (and maybe other nearby buildings) to hear it.$\endgroup$
– vszJun 21 '18 at 20:17

$\begingroup$I know essentially nothing about this stuff... but I'd have thought a 5.7x28 would be the last choice if you had a solid object in the way. Sure, it can defeat soft body armour (not plates), and apparently ignores laminated glass, but you need momentum to get through solid walls, and it doesn't have any (nor does a 22LR). If the bullet is to tumble in the body, there is almost no hope of going through the wall in a straight line. Random Commercial 5.7x28 vs. concrete block: note the lack of holes in the rear side. (no idea about military loadings though)$\endgroup$
– VisualMelonJun 21 '18 at 20:43

$\begingroup$@vsz Not necessarily. A .22 LR has low enough energy that punching through even a moderately thick piece of drywall is not going to be very loud.$\endgroup$
– Austin HemmelgarnJun 21 '18 at 20:48

$\begingroup$(All that said, maybe I just over-estimate how thick walls are, but the other answers suggest that it won't just be your average partition)$\endgroup$
– VisualMelonJun 21 '18 at 20:50

2

$\begingroup$@VisualMelon For drywall, you don't need much momentum, especially with the typical forward profile on most 5.7x28 rounds (drywall is soft enough that it just kind of gets pushed out of the way for small rounds like that). As far as tumbling in the body, that's a result of the shape, you lose the spin stabilization from the rifling when you get past a centimeter or so into soft tissue, but you still have lots of forward momentum (these things are still deadly at 1.5km, just not accurate enough to hit anything), and thus you tumble just like you would without the rifling because of the length.$\endgroup$
– Austin HemmelgarnJun 21 '18 at 20:54

Its incredibly far fetched and completely unnecessary but yes it's possible. A .50 caliber round will go straight through a brick wall and kill whatever is on the other side, but its trajectory will be altered somewhat but seeing as the victim will only be a foot or so away from the wall its a guaranteed kill, 5.56 weapons can be silenced as can 7.62 weapons, both can go through brick walls but with more trajectory change.

But realistically if they were able to get inside next door, and that close in general, a hit-man would probably just pick the lock and kill him in the middle of the night and to be fair a knife is as silent as is possible.

the assassin is more likely to leave evidence that can be linked back to him if he stays next door for such a length of time, whereas picking the lock in stab out is unlikely to leave much for the authorities to go on,

or when the victim is home just walk in next door plant a few kilos of plastic explosive and walk out, when a safe distance away flip the trigger cover (any good detonator MUST have a red trigger cover!!!) and press away, removes most if not all evidence the assassin may have left behind in that brief time

$\begingroup$Could you include references? I think those strictly real-world questions are an exception here. Basically they are answered by stating a bunch of facts and not by logic/creativity/common sense as many other questions here, so it would be nice to know if your facts are trustworthy$\endgroup$
– Raditz_35Jun 21 '18 at 15:14

$\begingroup$Why enter the target's home if you're using explosives? A shaped charge with ball bearings next to the dividing wall should go straight through. If placed at waist height for a standing person, that's a guaranteed kill$\endgroup$
– nzamanJun 21 '18 at 15:29

$\begingroup$Why even mention explosives? The original question makes no indication that the assassin even has access to plastic explosives.$\endgroup$
– AnketamJun 21 '18 at 15:46

1

$\begingroup$@Anketam i was offering an alternate suggestion, the questions states the killer is an assassin, therefore they are a professional, or at least attempting to be, therefore it is not unreasonable to assume they could acquire some. just as it's not unreasonable to assume he could acquire a knife..., and he did say is this setup reasonable, and explosives are just as unreasonable as shooting through a wall while a chap is doing his business$\endgroup$
– Blade WraithJun 21 '18 at 15:52

I'd like to suggest a variant with a bit more enhanced realism, in that he doesn't have to take any chances or make guesses (pro assassins would hate an uncertainty!), and also keeps very closely to the original idea, but addresses a few points mentioned in other answers/comments, if it doesn't spoil the intended plot. Also as much drama, and perhaps able to keep a reader guessing even more

Most toilets/bathrooms have a sink to wash ones hands. The sink has some interesting features for an assassin behind a wall - it's more often towards a far end if there's space, and it almost inevitably has a mirror (either over the sink, or if not, then a full size wall mirror as in some disability-friendly toilet/bathrooms). The mirror is what we want. It's separate from but flat against the wall (or flat against the sink's back panel/wooden units), and it's quite large.

Imagine the current plan. Marking an X on a wall and hoping the target isnt leaning forward, reaching sideways, holding their head bowed, reaching for thre paper or their trousers .... and if he misses and the target drops down low, how to figure his aim for a second finishing shot? No, the guesswork is far too much for a skilled assassin. But he wants to shoot through the wall anyway.

Can you see where I'm going?

Instead of a hole by the toilet or a blind shot through the wall, the assassin first locates the mirror in the bathroom. This should be fairly straightforward for the kind of assassin in the question. He could:

use a cheap metal/pipe detector as sold by any DIY chain, to locate and trace the sink pipework

use a long range view with binoculars, somewhere he couldn't shoot from but could view from

somehow gets a description from a past visitor/tradesman/associate

figures from the floor plans that most apartments have the same layout which pretty much forces the toilet in one corner and sink on the opposite wall

drills a tiny pilot hole somewhere else, like at floor level under the toilet/sink, and passes a fiberoptic size tiny fisheye lens through it to view the layout. (My favourite of these!)

He/she then carefully cuts/drills away a fair size hole behind the mirror of the sink, stopping when close to breaking through the wall to go really slowly. This isn't hard because walls in any specific building are almost always similar thicknesses. Noise isn't a problem - from time to time anyone's neighbour will do building work and everyone will assume that's all it is. So he doesn't need to know when the target is out or in a meeting. He might use an ordinary cheap angle grinder with a diamond blade, as used by any DIY/builder, instead of a hammer/chisel, because this cuts the wall away without putting any pressure/force on it. It would take maybe an hour or two to do it. So when nobody's around to hear, he can expose a sizeable area of the back of the mirror through the wall, like a rough circle 4 - 6 inches across, and it can't be detected.

And now, remember one key thing about mirrors. You know the one way mirror beloved of police interview rooms in thrillers? All that a one way mirror is, is a half silvered mirror with the "hidden" side in the dark. The person on the lit other side only sees their own reflection.

So your assassin turns off the lights in the next apartment, and with a fine abrasive, gently rubs the back of the mirror through the wall, stopping once he can slightly see through any part of it. If his rubbing is careful and fine (and he's practiced it of course!) then he's now converted his target's toilet/bathroom sink mirror into a makeshift one way mirror. There's no sign of it (who looks hard at their sink mirror?), and it's dark his side anyway.

But when the target comes in and sits down, he has a perfect sight for his hit, down the barrel of his gun, of the target with trousers round ankles on the toilet (or standing stationary looking at it), and all that lies between them is a thin sheet of glass. Even if the target moves, the hole is large enough to see everything and adjust his aim sharply downward. And being the other end of the bathroom, its a great vantage point.

He might figure as a fallback plan, even if the toilet location was wrong (they moved it in that bathroom), this will give him a good shot anyway, when the guy washes his hands, even though toilet is preferable because of immobility and trousers obstructing movement.

$\begingroup$Turning the mirror into a one-way-window like this holds a really really serious likelihood of inadvertently cracking the mirror and giving away the plan early, even if you know exactly what you're doing it's probably much too great a risk. That said, it only needs to remain undetected for a minute or so, the first time the person enters the bathroom they'll look at their cracked mirror and think "how did that happen?" and make a note to get it fixed/replaced. then they'll carry on as normal at which point it becomes moot because the assassin shoots them.$\endgroup$
– RuadhanJun 22 '18 at 10:35

2

$\begingroup$I've actually done part of this (not the assassin-ey bit I should say!). I've had to cut away a section of brick wall from one side without disturbing the neighbouring decor on the other face. That's how I know which tool works. You can cut thin slices within 1/4 inch of the other side, quite easily if careful, then use a screwdriver and they just break off and fall leaving a hole. Then use a small rounded carbide tip to wear out through to the far side of the wall. It worked fine, you can feel when you get through and avoid any pressure, especially as the mirror isn't attached and no impact.$\endgroup$
– StilezJun 22 '18 at 11:04

$\begingroup$I'm really curious what the context was for this :) But okay, I'll defer to experience on that one! The real question is how easy it is to scrape away the back of the mirror without cracking it,$\endgroup$
– RuadhanJun 22 '18 at 11:40

2

$\begingroup$I needed to check for a structural repair if an area was one or 2 bricks thick, but the other side was a neighbours. I could see exactly how thick the bricks in other walls were and figure a place which would be unnoticeable. With permission, I cut tiny slivers to get within a fraction of one brick deep, and gentlllllly wore a tiny pinprick hole through the last millimetre or two, to make a pinprick opening behind the stairs. I put a thin needle in the hole, probed if it hit plaster, a second brick, or empty space, then cemented up the hole again. It worked perfectly. And that's the story.$\endgroup$
– StilezJun 22 '18 at 13:16

2

$\begingroup$Given the goal is nothing more than target-ID and lining up the shot, rather than any sort of long-term observation or spying, I suspect it's overkill in the extreme to go this far :) you could achieve similar results with a pinhole in the wall and a fish-eye fibre-optic thread. Were it me, my weapon of choice for this would be the fibre-optic pinhole in combination with a compressed-air needle-gun. Almost silent, with the advantage that it doesn't blow a several inch across hole in the wall. just a pinhole in the wall that you drill before-hand and block up afterwards.$\endgroup$
– RuadhanJun 22 '18 at 14:07

I can't speak to your method of target acquisition, but as evidenced by this video, almost any firearm will massively over-penetrate drywall or Sheetrock.

I know it's a short answer, but I don't have that much more to say. Nearly any gun will go through the wall, and I don't see why you couldn't figure out where to aim with a detailed floor plan. All that said, if you're okay with B&E and Chloroform, why not just skip the elaborate plan and chloroform the target directly?

$\begingroup$have the assassin be female, walk up to the target in a bar and say, excuse me does this smell like cholorform to you? it won't kill him outright instantly but might be funny$\endgroup$
– Blade WraithJun 21 '18 at 15:07

5

$\begingroup$@BladeWraith If we're going for comedy, have the female assassin invite herself into the targets home in a skimpy dress. From the neighbors POV all thumps, bumps, and things getting knocked over make it sound like they're having a very good time.$\endgroup$
– UIDAlexDJun 21 '18 at 15:14

$\begingroup$very true, preferably to the benny hill theme tune going at the same time, or go the other way and go full basic instinct and sleep with the target then stab with an icepick, just so you could have that glorious interview scene again$\endgroup$
– Blade WraithJun 21 '18 at 15:40

$\begingroup$The site you're looking for for definitive info on shots thru walls is theboxotruth.com$\endgroup$
– seizethecarpJun 22 '18 at 21:09

It is possible, but there are some nuances you might add if you are into exposition and tangents (like Neal Stevenson, for example).

The wall is likely to be an extra-thick wall. Not with more layers of covering, but with greater distance between the two layers. This gives a greater internal cavity space for plumbing to be run. When constructing the thicker wall with standard studs, the construction is often done as two half walls, each with its own header, footer, and studs. The studs on one side often do not align with the studs on the other, so even though a stud finder could keep the assassin from firing directly into the stud on the assassin's side, the weapon could still be deflected or absorbed by the stud on the victim's side.

With so many pipes inside the wall (drain, vent, hot supply, cold supply, perhaps greywater supply) the assassin would want to avoid hitting them, as they could deflect a weapon.

How are the walls finished? Ceramic or stone tile would be a greater impediment to a bullet than would gypsum wallboard. The assassin could manually penetrate the wall on the assassin's side, which would give the assassin information about the internal structure.

Because the bathrooms are back-to-back, they probably share a drain pipe. With the right robotics, the assassin could approach from the neighbor's toilet, navigate through the trap to the common drain pipe, cross to the victim's toilet, through that trap, and proceed with the mission. Projectiles, explosives, injections, gasses, poisonous spiders, and other attacks become possible.

$\begingroup$In North America, "Green board" or cement board is used in place of sheetrock for bathrooms due to its greater density and water resistance. While it won't stop a bullet, it will take more of the bullet's energy to penetrate. Cement board is harder and denser still, and sometimes used due to it's greater strength and waterproofness.$\endgroup$
– ThucydidesJun 22 '18 at 2:22

$\begingroup$There's a viral story making the rounds this week about a plumber whose snake took a wrong turn, came out in another bathroom, and grabbed the shower curtain.$\endgroup$
– arpJun 24 '18 at 5:12

$\begingroup$When the OP said shoot through a wall, I don't think they meant destroy the whole building with 30 rounds of .50 AP. - A "classic SAS operation", indeed: express delivery of 48oz of lead at 3000 fps.$\endgroup$
– MazuraJun 21 '18 at 18:28

1

$\begingroup$@Mazura I hoped that "even if you miss your shot enough wall shrapnel rips through the toilet to incapacitate anyone inside" would answer the OP's question :p.$\endgroup$
– DemiganJun 21 '18 at 22:08

If the assasin has gained entry into the apartment, it would be easier to rig the toilet to explode, rather than mess around drilling holes. (This idea is stolen from Chris Ryan's book, Hunter Killer). An explosive device contained within the cistern, along with a small web-cam, will ensure positive identification of the target, and a very high chance of success. The cistern would act like a Claymore, ripping the toilet's occupant to shreds, and providing the device is large enough most evidence will be destroyed. (Shooting the guy might tie the assassin there due to ballistics and other forensics found, and has to be there onsite for the kill, but a remote explosive allows our assasin to be on the other side of the world, with minimal evidence tying him to the scene)

As many people noted, it is far too elaborate and has too many unknowns (like hitting unexpected pipes or offset studs).

Some simpler methods might include:

A sniper rifle, binoculars and a thermal imager. The assassin across the street (no need to make entry into the building) watches through a window to see the movements of the target. When the target is PID ((Positively Identified) and going into the washroom, the assassin takes up the sniper rifle with thermal imager and lays onto the area where the toilet is (this can be determined by the floor plans). Under the right conditions, the ghostly image of the target comes into view and the assign snipes the target as the target settles on the toilet.

Image using a FLIR (Forward Looking Infra Red) device. A thermal imager would provide a similar image

The assassin rigs a weapon to a quadcopter drone and flies it in through the bathroom window. This can be as simple as a hand grenade sized bomb, or even an actual pistol or PDW type weapon.

Surprise!

If you really want to have an elaborate assassination scene, follow the "Wrath of God" team which tracked down and killed Black September members after the Olympic Massacre in Munich in 1972. According to the book "Vengeance", by George Jonas, the team found Mahmoud Hamshari in Paris, snuck into his apartment and placed a small explosive device in the handset of his telephone (this was 1972, so things like cell phones had not yet been invented). When he came home, the Mossad agent simply dialled the number, asked if this was Mahmoud Hamshari and when he received confirmation, triggered the device remotely. Since the explosion was being held to the side of Hamshari's head, Mahmoud Hamshari died of his injuries.

For an up to date version of the phone bomb, the assassin looks for a shot through a window. Ideally the victim will be sitting with their back to the window, and the assassin can call on his cell phone or contact him via social media if the computer is against the wall and the victim sits with their back to the window. The assassin can activate the phone of computer camera by remote control or hack to confirm the victim is actually there, and even (gruesomely) record the effects of his own work once the shot is made. (Note. understanding how this is done is not the same as condoning hacking people's computers. Use this information to secure your own machine). Update to add: Why shoot if you can get enough explosive hidden in the phone case or computer/laptop case? The assassin simply uses the camera or microphone to PID the target, then detonates the device with the target in close proximity.

Of course depending on the skill of the assassin, the notoriety of the victim and the needs of the body sanctioning the hit, the assassin could simply fill the basement of the building with a large explosive device and level the entire structure, with the trigger being the flushing of the victim's toilet.....

I feel this is needlessly complicated, if your scope is "figure out where they are through a wall and shoot them" then using either a FLIR unit or some form of penetrative radar imager to identify them will probably be sufficient.
The radar imager has the advantage of being able to also see any obstacles inside the wall such as pipes and structural beams that might stop or deflect the shot.

Neither option requires physically gaining entry to the apartment or takes longer than a few minutes of setup and packup. The getaway being a vital component of the plan because when the neighbours hear a loud gunshot from inside their building then the police won't be far away.

Others have covered the process of locating the person, I would think a fine drill and a fiber-optic cam close to the floor/ceiling would be good enough to inspect the room and locate the occupant.

I would consider a shotgun as the weapon of choice, possibly a suppressed 12 gauge like this. It's nothing like silent but you could theoretically use a 20 gauge in a similar configuration without being in sci-fi territory. Either way with buckshot at < 3 feet you've got a little more forgiveness regarding accuracy because a bunch of shot will carry more stopping power and has more chances to ruin something important.

Alternatively as someone suggested a suppressed SMG, you could fire a burst from head level down and likely get something important, subsonic loads or hollow points would do fine since a burst will clear the drywall anyway.

Finally, consider basically a spike, like an estoc, smallsword or even a pointed bit of rebar with some duct tape on the back. If you grant that the person can locate the target effectively, you may as well just remove all wall material except the drywall on the target's side, make a small divot to rest the point. Then wait for their arrival and brace the blade against the wall, when they're in position you just lean into a heavy thrust, you'll easily clear the wall and strike the target in the head, neck, torso. Depending on the arrangement you've definitely stricken them and possibly/probably killed them, possibly you've pinned them in place. Either way it's far quieter as regards the weapon and may indeed be quieter from the target if you get it where you need to.

After gaining access to the neighboring apartment and quietly neutralizing its occupant by means of chloroform, the assassin enters the room.

Other answers have pointed out a lot of problems with neutralizing the neighbour, most/all of which suggest entering the target's apartment is the superior choice.

If you really want this plot device, I'd suggest making the neighbouring apartment vacant instead. Occupants at work, away on vacation, unit currently for lease, renovations, bug bombs, loads of ways to do that.

If you've gone to all that trouble, why not just cut a small hole in the wall?

That way, you can establish if there are any pipes in the way, see that your target is where you expect them to be (guys pee standing up, which might throw off your sighting) and then use a silenced .22 without losing any velocity from going through the wall.

Consider some more creative approaches to taking someone out while they're on the commode...

You could feed your target 3000 cheeseburgers so they die like Elvis did... on the toilet.

Put a cobra in the toilet so they get nailed when they sit down (and leave a memorable mental image for your readers to remember the next time they have to go)

$\begingroup$The victim could be sleeping on their side, on the couch, staying out late or at another house altogether. Unless you can see them directly, the bed is not a great bet.$\endgroup$
– MuuskiDec 3 '19 at 17:29

As I believe others have said, inducing food poisoning seems unnecessary. The victim will go to the toilet sooner or later in any case.

The assassin would have the problem of gaining access to the neighboring apartment. A simpler solution is to ring the doorbell. While the suspicious future victim might use a peephole, the assassin can simply fire through the door. To keep the firing through walls scenario, you may wish to specify that the neighborhood is unsafe enough that all the front doors are heavy reinforced doors without peepholes, and that the victim has a video interphone system.

If being in the neighboring apartment is interesting from a plot point of view, I have a more geeky method to suggest. It is possible to use radio waves to detect humans through walls, see recent MIT research. The equipment could probably be disguised as a normal computer or two. Rent the neighboring apartment, calibrate the system, wait until the victim is stationary (sleeping, for example), and then fire.

Then run to ring the victim's doorbell, crying that you're very sorry but you were cleaning your gun and it went off and you hope nobody was home . . . frightened . . . hurt . . . dead? Oh no!

This material is very fragile and thin. you just have to locate the wall without plumbing. Bathroom walls are made of Pladur -N, a water repellant version that is only 1.2cm thick, but has a surface absortion ratio of 180gr/h * m2.

You could probably just fist-smash trough the wall.

Source? my whole apartment building is a big concrete shell ( only columns and floors, covered on the outside by bricks. every single other inner wall is Pladur/Durlock, be it single (bathrooms & normal rooms) or double ( kitchen/dining room walls that touch with the nighbors).

At worst you'd need to go trough 2.8cm of Pladur/Durlock + a 1.1mm aluminium structural vertical joint that is sometimes used to hang stuff up ( like the shower head's support). I bet most commercial hunting guns can do that easily, so for an assasin's high-cal pistol would be an easy task.